In the past, photosensitive resin printing plates have been known, such as flexographic printing plates obtained, using a printing material including a photosensitive resin layer, by exposing the photosensitive resin layer through a masking film as a negative film having a desired pattern, and then removing unexposed portions of the photosensitive resin layer by washing.
Masking films used in the production of such a photosensitive resin printing plate are generally produced by photographic methods using silver salts or various printing methods such as a laser beam printer. However, these methods cannot deal with large-sized masking films. In the case where large-sized masking films for use in the printing of newspapers or the like, for example, are required, two or more masking films are used, and they are joined together in an aligned manner. In addition, in a photographic method, although resolution is excellent, there are drawbacks in that the operation must be done in a special dark room, and, further, a number of chemical treatments are required, whereby the operation is complicated, for example. Meanwhile, in a printing method, the pattern can be formed directly by a printer using digital data, so the operation is simple. However, in a printing method that uses a laser beam printer offered by XANTE (U.S.), there are drawbacks in that chemical treatments are required in order to increase the light-blacktop properties of a masking film, and also that the resolution is poor.
Meanwhile, in the field of printing plate production, as a technique that uses a laser beam, for example, the following disclosures have been made: a resin printing plate having a depressed portion for printing, which is formed, using a printing material having a resin layer on the surface thereof, by irradiating a desired portion of the resin layer with a laser beam to allow the resin layer to sublimate or decompose (see, e.g., Patent Document 1); a printing plate formed, using a resin printing material having on a substrate an ionizing-radiation-sensitive resin layer and an ionizing-radiation-shielding layer, by first removing the ionizing-radiation-shielding layer corresponding to printing image with a laser beam, then subjecting the ionizing-radiation-sensitive resin layer corresponding to such areas to ionizing radiation, and subsequently developing the ionizing-radiation-sensitive resin layer (see, e.g., Patent. Document 2); etc.
However, these techniques are techniques of producing a printing plate by irradiating a resin printing material directly with a laser beam rough no masking film, and are not techniques of producing a masking film.
Under such circumstances, the present inventors have previously proposed a member for masking films suitable for the production of a masking film to be used in the production of a photosensitive resin printing plate such as a flexographic printing plate or the like, which provides a masking film that does not require a chemical treatment, allows high resolution to be achieved by etching with a laser beam, and is applicable to large-sized plates, as well as a process for producing a masking film using the same (see e.g., Patent Document 3).
Further, as an improvement of such a technique, they have proposed a member for masking films, including a base film transparent to ultraviolet rays and, on one surface thereof, a 0.1- to 30-μm-thick ultraviolet-shielding resin layer whose color can be eliminated by irradiation with a laser light beam and which has an ultraviolet transmittance of not less than 0.1% before irradiation with a laser light beam. The ultraviolet-shielding resin layer contains, as a blackish pigment, a carbon black having an average particle size of 20 to 50 nm, a specific surface area of 70 to 150 m2/g, and a center-line-average surface roughness Ra of 0.1 to 3 μm (see, e.g., Patent Document 4).
A masking film obtained using this member for masking films has favorable characteristics as above. However, because the ultraviolet-shielding resin layer is a single layer, the masking film is prone to defects, such as coating nonuniformity and pinholes, as well as scratches due to abrasion the resin layer surface, etc. Thus, in areas whose color has been eliminated by a laser beam, such problems may directly appear as a irregularity in light transmittance, or pinhole areas may be exposed, which may make it impossible to accurately produce a printing plate. Therefore, it has always been sufficiently satisfactory.
Further, in order to efficiently perform accurate processing with a laser, there has been a demand for a member for masking films, which has an ultraviolet-shielding resin layer with reduced thickness or has fewer defects in the production process, such as coating nonuniformity and pinholes.